1. Field of the Invention
This invention relates to power transmissions, more particularly to continuously variable, traction transmissions especially those for motor vehicles.
2. Description of the Prior Art
Various transmissions that produce a continuously variable, stepless range of speed ratios have been developed. Usually these transmissions include a variator for producing all the speed ratios. One type of variator includes variable diameter sheaves driveably engaged by a drive belt or chain, which changes its radius of contact on the sheaves as the sheaves move axially. Usually a gearset is required to produce reverse drive with a belt drive variator of this type.
Toroidal traction drive transmissions use either half-toroidal or full-toroidal traction drives, which typically employ dual cavities for maximum efficiency. The dual cavity units have two input discs and two output discs each having a toroidal or partial toroidal shape. The output discs are typically located centrally between the input discs. Each input disk is engaged with an output disc through traction rollers. The angle of the rollers is varied to change the drive ratio between the input and output discs.
Toroidal variators are used to produce neutral, reverse and slow forward drive by combining a direct output from the vehicle's engine and an indirect output from the variator, which is reversed in directional sense. In this way the transmission produces either a low range of reverse speed ratios, a low range of forward speed ratios, and geared neutral, in which the engine is rotating, but the variator output is rotating in the opposite sense and the combined output is zero.
European Patent EP 0 078 124 describes a transmission that includes an input shaft for connection to an engine crankshaft, a rolling traction toroidal race, continuously-variable variator driven by the input shaft, and a simple epicyclic gearset comprising a sun gear, planetary gears on a carrier, and a ring gear. The gearset combines the output of the variator with the output of a step-up or step-down gear connected to the input shaft. The output of the variator is connected directly to one part of the epicyclic gear, the input shaft is connected via a step-up or step-down gear with another part of the epicyclic gear, and the output of the transmission system includes a gear, connectable by a first clutch to a third part of the epicyclic gear and by a second clutch to the output of the continuously-variable-ratio transmission.